In the copending application of Dennis R. VerDow, Ser. No. 90,470, titled "High Strength Cordierite Ceramic," filed Nov. 1, 1979, there is disclosed a cordierite-type ceramic which has a wide firing tolerance, unusually high strength, and which among other uses is especially suitable for use in freezer-to-oven utensils and dinnerware.
This application relates to a glaze which is particularly adapted for use on cordierite ceramics of the type there disclosed, although it can also be used on other fine ceramics.
Cordierite, 2 MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, corresponds to the oxide composition 51.3% SiO.sub.2, 34.9% Al.sub.2 O.sub.3, and 13.8% MgO. Cordierite ceramics (i.e., ceramics having a substantial content of cordierite crystals) possess a low coefficient of thermal expansion, in the range of approximately 14-40.times.10.sup.-7 /.degree.C. Notwithstanding the advantageously low expansion, however, cordierite ceramics have found only limited use in ceramic whiteware for cooking and dining use. One reason for this has been the limited glazability of such ceramics.
Whiteware is usually glazed in practice, in order to provide a protective surface coating which is hard, resistant to attack in its intended use, and impermeable. Typical glazing practice requires that the glaze be of lower expansion than the ceramic body onto which it is applied, so that the glaze will be in compression rather than tension. Being of low expansion themselves, cordierite ceramics require glazes of very low expansion, and this constitutes a rather severe constraint in practice; relatively few glasses are known which have expansions lower than cordierite ceramics. Moreover, the problem is further complicated by the fact that many glasses which might be useful require melting at temperatures above the practical firing limits or cordierite ceramics.
Self-glazing techniques are known for cordierite bodies, wherein a glaze "skin" self-forms on the body during firing of the body itself, without application of a separate glaze. However, it is difficult to control the physical properties of self-glazed cordierites, and insofar as I am aware the technique has not developed commercially.
Glazes have heretofore been disclosed for use on cordierite-based low expansion ceramics, but they have had undesirably low surface gloss and durability, and they have been translucent or opaque rather than transparent. Eppler U.S. Pat. No. 3,840,394 discloses a glaze for use on cordierite ceramics, wherein the glaze contains crystals of barium oxide compounds such as celsian. Eppler's glazes comprise 6-65 wt.% BaO, 0-12% MgO, 0-15% Na.sub.2 O, 9-35% Al.sub.2 O.sub.3, 18-65% SiO.sub.2, and 5-30% flux. These glazes depend on a relatively high content of BaO in relation to the other crystal forming components: the ratio of the total of SiO.sub.2 +Al.sub.2 O.sub.3 +MgO, to BaO is less than 15:1. As fired, the Eppler glaze has a matte to moderate gloss characteristic. The Eppler glazes are described as being translucent or opaque.
The Li.sub.2 O-containing glazes, wherein the lithia content leads to the formation of low expansion lithia-alumino silicate crystals such as Beta-eucryptite and Beta-spodumene, have previously been suggested for use on cordierite ceramics. Those glazes are highly crystalline, and the index of refraction of the Li.sub.2 O-containing crystals differs substantially from that of the glassy matrix around them. As a result of this difference in refraction, such glazes are opaque and they lack the brilliance or clarity which in many cases is so desirable to provide the appearance of fine china. Other opaque glazes have been suggested for cordierite bodies, but they have had undesirable characteristics including application difficulties, limited firing ranges, variable gloss and limited decorating possibilities.